<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>红绿黄灯倒计时</title>
    <style>
        .box{
            width: 250px;
            height: 52px;
            padding: 15px 30px;
            border: 2px solid #ccc;
            border-radius: 16px;
            margin: 0 auto;
        }
        .box.count{
            width: 60px;
            color: #666;
            font-size: 280%;
            line-height: 50px;
            padding-left: 6px;
            margin-left: 5px;
            border:none;
        }
        .box div{
            margin-left: 5px;
            float: left;
            width: 50px;
            height: 50px;
            border-radius: 50px;
            background-origin: 1px solid #666;
        }
        .gray{
            background-color: #eee;
        }
        .red{
            background-color: red;
        }
        .yellow{
            background-color: yellow;
        }
        .green{
            background-color: #26ff00;
        }
    </style>
</head>
<body>
    <div class="box">
        <div id="red"></div>
        <div id="yellow"></div>
        <div id="green"></div>
        <div class="count" id="count">00</div>
    </div>
</body>
<script>
    var lamp={
        red:{
            obj:document.getElementById('red'),
            timeout:30,
            style:['red','gray','gray'],
            next:'green'
        },
        yellow:{
            obj:document.getElementById('yellow'),
            timeout:5,
            style:['gary','yellow','gray'],
            next:'red'
        },
        green:{
            obj:document.getElementById('green'),
            timeout:35,
            style:['gray','gray','green'],
            next:'yellow'
        },
        changeStyle(style){
            this.red.obj.className=style[0];//通过className获取对象的class，通过改变class将样式改变
            this.yellow.obj.className=style[1];
            this.green.obj.className=style[2];
        }
    };
    var count={//倒计时对象
        obj:document.getElementById('count'),
        change:function(num){
            this.obj.innerHTML=(num<10) ? ('0'+num):num;
        }
    };
    var now=lamp.green;//通过lamp地下获取到green对象
    var timeout=now.timeout;//获取到green对象中的timeout属性
    lamp.changeStyle(now.style);//第一次执行时需要将绿色添加样式
    count.change(timeout);

    setInterval(function(){//设置定时器，每一秒执行一次函数
        if(--timeout <=0){//每执行一次对象的时间减一
            now=lamp[now.next];
            lamp.changeStyle(now.style);
        }
        count.change(timeout);
    },1000);
</script>
</html>